The Polycomb group (pc-G) proteins are suggested to function as a complex or complexes to maintain a repressed state of numerous developmental important genes, based on genetic studies in Drosophila. Several mammalian homologues of Pc-G proteins have also been isolated recently, including a proto-oncogene, bmi-1. Little is known about the mechanism(s) through which the Pc-G proteins function to repress transcription, which could be through alteration of chromatin structure or through direct inhibition of the activities of transcriptional activators. The proposed research will use biochemistry analysis as well as genetic studies in order to understand the molecular basis of this aspect of gene regulation and the role it plays in the determination process. Understanding the mechanisms through which an activated or repressed state of transcription is initiated and maintained will improve our knowledge on embryogenesis as well as tumorigenesis. There are three specific aims for this proposal. 1. To study the effects of the Pc-G proteins on the activities of transcriptional activators in cell lines stably transfected with LexA- and GAL4-dependent reporters, GAL4-fusion transactivators, and LexA-fusion polycomb group proteins. 2. To determine the biological relevance of the LexA-Pc-G chimera proteins and their effects on transcription in transformed Drosophila lines. 3. To purify the Pc-G complexes from Drosophila embryos and study their effects on transcription in vitro.